In recent years, electric vehicles and plug-in hybrid vehicles have become widespread. In these vehicles, a storage battery for supplying power to a motor when a vehicle is running is mounted. When the storage battery is charged from a commercial AC power supply, a charging apparatus having a high conversion efficiency and including a current resonance type converter having a function to insulate the AC power supply and the storage battery. Generally, the current resonance type converter can easily realize high efficiency. However, it is difficult to realize both a constant current characteristic at low voltage and low current at the time of startup of a charging apparatus and a constant current characteristic at the time of normal operation. For example, PTL 1 discloses a method for improving compatibility between the constant current characteristic at low voltage and low current at the time of startup and the constant current characteristic at the time of normal operation by adding a switch for separating an auxiliary winding circuit and a main circuit.